Okay, MT-Moderate is installed. The basic gist of the plugin is that it takes comments on entries that are more than 7 days old and automatically chucks them into the new comment moderation feature of MT, the idea being that comment spammers generally go after posts that are older since those have the most potential to boost Google juice.
MT-Moderate is doing what it's supposed to...If you submit a comment on a newer post, it will go right through, but it automatically goes to moderation if you comment on an older post. This is pretty cool, however, it doesn't address what I initially installed MT-Keystrokes for, which is to automatically nuke comments that are submitted by bots. I still have to go into comment moderation and manually kill spam.

I don't know if MT can currently do this, but here's an idea: If you could set MT to automatically purge comments that stay in moderation for more than 7 days, that would automate a task that is a giant pain in the ass for me. If MT would do this, comment moderation would consist of manual approval of comments on old posts, weeding out the small number of spams that come to new posts, and then simply letting spam comments on old posts rot away. Does anyone know if MT can be set up to do this? If not, that's a plugin idea for an enterprising programmer.

This arms race totally blows. It's so frustrating to think that every few months, I have to go out and find something to handle the latest tactics employed by the comment spammers. It's even suckier for the plugin developers who take their own time to develop spam-nuking countermeasures and then find that they're essentially useless a few months down the road.

I think the key to this is to get some cooperation from Google on the whole "nofollow" thing. (Dunno what the current state of this is.) Basically, this approach would de-incentivize blog spammers by giving bloggers the power to tell Google to avoid following certain links. Nofollow = No Google Juice.

Here's one plugin I'd like to see: Something could automatically weed out comments by eliminating ones that link to the same base URL twice within an adjustable length of time (default would be 24 hours). That way, comment spammers would be able to get in only one comment per 24 hour period that linked to the same affiliate. This wouldn't work for everybody, but for blogs like mine that get 3 or 4 legit comments per 500 spams, it would save a lot of time. (Especially if it worked in conjunction with existing plugins.) I notice that most of my spam comes from a variety of IP addresses, but they tend to link to the same pages. True, a persistent spammer could set up multiple domains that redirected to the same page, but that requires an additional investment. Such a plugin would provide a negative economic incentive and I think it would cut back on a lot of the spam.